Foul Toilet Smell

We follow of the "If it's yellow let it mellow..." (urine gets no flush or a "half" flush) system and have never had a problem with it until we until we moved recently.

In this house anytime urine sits in the toilet for more than a few hours, a strong sewage smell comes from both of our toilets. We accidentally left some "yellow" in one and went out of town for 5 days recently only to come back to a toilet with a brown film over the top and an awful smell filling the bathroom and attached bedroom.

I don't notice any smell if the toilets are always flushed. The toilets seem well seated and there is no noticeable water damage around them.

in the previous house there was minor leakage letting new water into the bowl. That diluted the urine and eventually made it practically disappear. In the new house, the toilets have no leakage between the tank and bowl, so the urine acts as any stale urine does. It turns bad.

I think David may be onto something.
If the flappers on the old toilets were bad, you may have been having clean water go through.

Urine does go bad, or maybe the word is ferment.

That's why waterless urinals smell so bad.
You can smell them before you enter the mens room.

You may want to consider a dual flush toilet.

Here is a toilet drain from a home where the toilet was rarely flushed. The fairly new toilet was thrown away. Not flushing the bowl meant that salt was depositing in the trapway and closing down. Here is a picture of the drain below the toilet and the salt deposits.

It is possible your new house has older toilets and there may be bacteria that reacts strongly with the urine. This bacteria may be coming from the part of the toilet trapway that you can't see but comes in contact with the water/urine mix when the bowl is full via a non glazed trapway.

Toilet replacement may correct your problem.

If you decide to purchase new toilets, Be sure the trapway is glazed and check out the toilet section of this forum B 4 deciding on what brand.